1. Field of the Invention
This invention relates to a method for producing fatty acid alcohol ester useful as a substitute fuel for light oil, and more particularly relates to a method for producing fatty acid alcohol ester together with highly valuable glycerol as a byproduct.
2. Prior Art
Due of lower ignitibility and higher pour point of such vegetable fats or oils compared with those properties of light oil, they themselves are not useful as a substitute fuel for light oil.
On the other hand, fatty acid alcohol ester prepared by reacting the vegetable fats or oils as a starting material with alcohol is considered as a hopeful biodiesel fuel oil (hereinafter referred to as BDF) from a viewpoint of measures against global warming to control an increase in carbon dioxide on the earth.
A mixture of the vegetable fats or oils with alcohol in the presence of a catalyst causes an ester interchange reaction to substantially change the vegetable fats or oils to fatty acid alcohol ester, thereby glycerol and fatty acid soap being formed as byproducts (see, JP-A No. 2000-143,586, JP-A No. 2001-524,553 and “Biodiesel Production and Quality” (2002), a leaflet of U.S. National Biodiesel Board (NBB)).
The above mentioned reaction comprises, for example, formation of fatty acid alcohol ester and glycerol as a main reaction and formation of fatty acid soap and water as a side reaction as shown in the following reaction formulas: 
Ignitibility (cetane number) and pour point of the fatty acid alcohol ester approximate to those of light oil and thus are sufficiently useful as a diesel fuel, i.e., a substitute fuel for light oil.
As an ester interchange reaction is so slow that it takes several hours to complete the reaction at ordinary temperatures and is carried out in a reactor of larger scale, the reaction is generally accelerated by rising temperatures, which requires higher thermal energy.
Further, a limit of rise in temperature is about 50 to 60° C., although the reaction rate can be accelerated with an increase in temperature, because volatile alcohol such as methanol is used.
Furthermore, even when the reactor is equipped with an agitator or line mixer to improve its agitation effect, it takes about 30 minutes to one hour to conduct the reaction.
The above mentioned alcohol or catalyst is sometimes used in an excessive amount to accelerate the reaction, however, unreacted alcohol, residual catalyst or excessively formed fatty acid soap should be removed from the ester product in the purification process, which would require scale up of the purification facilities, make the reaction system complex and increase the plant investment or running cost.
As has been described above, conventional production technology of fatty acid alcohol ester yields ester and glycerol of inferior purity as the reaction products due to an excessive amount of alcohol as one of starting materials and catalyst used in the reaction, thereby affecting the combustibility of DBF and exhaust gas composition therefrom and disturbing an practical application thereof. If the conventional production technology is employed as it is in the purification process, an enormous production cost should be necessary.
Considering the present status of this technological field, it is inevitable to develop technology for producing and purifying fatty acid alcohol ester at a greatly reduced production cost so as to spread it as a substitute fuel for light oil.